1. Field of the Invention
The present invention relates to a communication vocoder (voice coder). More specifically, the present invention relates to a tandemless vocoder.
2. Description of the Related Art
As development of digital mobile communication systems has significantly accelerated, vocoders suitable for respective mobile communication systems have also been developed. RPE-LTP (regular pulse excited long term prediction) vocoders developed for European GSM digital mobile communication systems were replaced by EFR (enhancement full rate) GSM vocoders, which were subsequently replaced by AMR (adaptive multi rate) vocoders for varying the data rates according to characteristics of wireless transmission intervals. In particular, the AMR vocoders have been adopted for the 3rd generation mobile communication system, that is, the European (asynchronous) IMT-2000 system, because of their excellent voice quality and performance.
QCELP (Qualcomm code excited linear prediction) vocoders using the CDMA method were developed for the North American digital mobile communication system, 13 kbps QCELP vocoders with 13 kbps data rates were subsequently developed, and EVRC (enhancement variable rate codec) vocoders were finally developed. The North American (synchronous) IMT-2000 system standardization committee has completed the SMV (selectable mode vocoder) standardization of the AMR vocoders.
In general, communication between vocoders using different methods is executed by decoding voice packets with one method into voice signals, encoding the voice signals through another method, and decoding the encoded voice signals, which is referred to as the tandem method.
FIGS. 1 and 2 show block diagrams of conventional tandem vocoders. As shown in FIG. 1, the tandem communication method from an A-type vocoder 10 to a B-type vocoder 20 is performed by a secondary encode/decode module 30. That is, the voice packets encoded by an encoder 11 of the A-type vocoder 10 are decoded into voice signals in the PCM (pulse coded modulation) format by an A-type decoder 31, the voice signals are encoded again by a B-type encoder 32, and the encoded voice signals are transmitted to a decoder 21 of the B-type vocoder 20. Because the encoding/decoding processes are performed twice, the tandem communication method lowers communication quality, increases delay time, and needs secondary encode/decode modules 31, 32, 33, and 34 in the intermediate communication area.
To solve the problems, a method shown in FIG. 2 has been proposed. As shown, packet converters 41 and 42 are provided between the A- and B-type vocoders 10 and 20. The packet converter 41 mathematically analyzes a bit sequence of the packet encoded by the encoder 11 of the A-type vocoder 10, converts the same into a bit sequence for the B-type vocoder 20, and transmits the bit sequence to the decoder 21 of the B-type vocoder 20. In a like manner, the packet converter 42 converts a bit sequence of the packet encoded by the encoder 22 of the B-type vocoder 20 into a bit sequence for the A-type vocoder 10. However, even though the mathematical analysis is very accurate, its voice quality is worse than that generated by the encode/decode process of the encode/decode module, and it additionally requires an analyzer, thereby generating a delay time.